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Did you perhaps install Microsoft Visual C++ 2017 and/or 2019 around the time your issue began? While investigating an issue of my own, I came across information at The latest supported Visual C++ downloads that has my head spinning because that's not how it used to be:

Note Visual C++ 2015, 2017 and 2019 all share the same redistributable files.

For example, installing the Visual C++ 2019 redistributable will affect programs built with Visual C++ 2015 and 2017 also. However, installing the Visual C++ 2015 redistributable will not replace the newer versions of the files installed by the Visual C++ 2017 and 2019 redistributables.

This is different from all previous Visual C++ versions, as they each had their own distinct runtime files, not shared with other versions.

Just a hypothesis, but your legacy version of Plex may have been partially broken by redistributable files that did not exist when it was developed. According to a Plex post about the same age as the last version to support Vista, it required Visual Studio 2015 with Update 1.